Separator for fuel cell, manufacturing method thereof, and fuel cell having such a separator

ABSTRACT

A lamellar structure graphite foil is used as a material for a separator for a fuel cell, and a hydrophobic layer is formed by impregnation on flow-field channels of the graphite foil. Such a separator is manufactured by forming the flow field channel by etching the graphite foil formed with the mask pattern thereon and forming a hydrophobic layer by impregnation. According to such a separator, performance of a fuel cell stack is enhanced and the manufacturing process of a separator is simplified.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage Application under 35 U.S.C. §371 ofPCT Application No. PCT/KR2004/001950 having an International FilingDate of Aug. 3, 2004, which designated the United States, which PCTApplication claimed the benefit of Korean Patent Application No.10-2004-0016162, filed on Mar. 10, 2004, the entire disclosure of whichare hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

Generally, the present invention relates to a fuel cell. Moreparticularly, the present invention relates to a separator for a fuelcell using a graphite foil, a manufacturing method thereof, and a fuelcell stack including such a separator.

(b) Description of the Related Art

As is well known in the art, a fuel cell produces electric power by anoxidation reaction at an anode and a reduction reaction at a cathode.The anode and the cathode are formed with a catalyst layer havingplatinum or platinum-ruthenium metal for accelerating the oxidation andreduction reactions.

At the anode, fuel gas (for example, hydrogen) is supplied thereto andis divided into ions (for example, protons) and electrons through theoxidation reaction. At the cathode, the divided ion bonds with areduction gas (for example, oxygen) to form water. Final products ofsuch reactions are electricity (i.e., electron movement from the anodeto the cathode), water (i.e., a chemical bond of hydrogen and oxygen),and heat. A fuel cell stack is usually provided with a cooling devicefor removing such heat.

The water formed at the cathode is usually formed as vapor or liquid,and such water is removed by a strong stream of reduction gas (oxygen orair) flowing at a cathode side.

FIG. 1 is a schematic sectional view of an exemplary fuel cell stackaccording to the prior art.

Usually, a fuel cell stack is formed by stacking a plurality of unitcells 100.

Such a unit cell 100 includes a proton exchange membrane 110 (forexample, a polymer electrolyte membrane). An anode 121 and a cathode 122are formed at both sides of the proton exchange membrane 110. The protonexchange membrane 110 and electrodes 121 and 122 form a membraneelectrode assembly (MEA) 130 by hot pressing. Fluid diffusion layers 125are formed to the exterior of the electrodes 121 and 122 of the MEA 130.

MEAs 130 of adjacent unit cells are separated and supported by aseparator 150. The separator 150 is formed with a flow field 151 forsupplying fuel gas (e.g., hydrogen, or methanol in the case of a directmethanol fuel cell) to the anode. In addition, the separator 150 is alsoformed with a flow field 152 for supplying oxygen or air as a reductiongas to the cathode, and also for exhausting water. A gasket 160 isapplied between the separator 150 and the MEA 130, for preventingleakage of gas/liquid flowing through the flow fields 151 and 152.

The unit cells 100 including the MEA 130, the separator 150, and thegasket 160 are stacked in series to form a high voltage. The stackedunit cells are conjoined by, e.g., current collectors and end plates 170disposed at ends thereof.

As can be understood from the above description, a separator in a fuelcell distributes reaction gases (i.e., fuel gas and reduction gas)through the fuel cell stack, separates a fuel gas (e.g., hydrogen ormethanol) and a reduction gas (e.g., oxygen or air), and electricallyconnects adjacent unit cells by providing an electron passage between ananode and a cathode of adjacent unit cells. In addition, the separatorhas a heat exhaust structure for exhausting heat produced by theoxidation-reduction reaction of the fuel cell stack, and providesmechanical strength for supporting the stacked unit cells.

In order to accelerate movement of hydrogen ions (i.e., protons)produced at the anode to the cathode through a polymer electrolytemembrane, the polymer electrolyte membrane should be hydrated to containan appropriate amount of moisture. The hydrated polymer electrolytemembrane prevents movement of electrons therethrough while allowingmovement of hydrogen ions.

When the polymer electrolyte membrane is not sufficiently hydrated, ionconductivity of the polymer electrolyte membrane is lowered, andtherefore performance of a fuel cell is deteriorated. To the contrary,when the polymer electrolyte membrane is excessively hydrated, smallpores forming a triple-phase boundary of reaction are blocked (which isusually called flooding), and thereby the reaction area of theelectrodes reduces, resulting in deterioration in performance of thefuel cell.

Therefore, in the case that the water formed at cathodes is not promptlyexhausted, reaction gas is not sufficiently supplied to the catalystlayer, and therefore performance of a fuel cell is deteriorated.

Many separators, including an exemplary one disclosed by U.S. Pat. No.4,988,583, have serpentine flow fields for fuel and reduction gases.This is mainly for utilizing a pressure drop along the flow fields forefficient exhaust of water formed at the cathodes.

The water formed at the cathodes is in the form of vapor, near the entryof reduction gas flow-field channel. However, as it flows through thereduction gas flow-field channel, it becomes of two phased, as mixedliquid and vapor. In this case, liquefied water drops fill the pores ofthe cathodes, and accordingly, the effective active areas of thecatalyst layers become reduced. In addition, liquefied water requires ahigh pressure of reduction gas for exhaust thereof.

Therefore, energy loss occurs by a pressure drop of reduction gasbetween entry and exit of flow fields, and reaction gas is much consumedfor stable realization of the reduction reaction at a high flow speed.Therefore, if water exhaust of a separator having serpentine flow fieldsbecomes more stable and efficient, it will promise a reduction of energyloss by the pressure drop of reduction gas between the entry and exit ofthe flow fields and reduction of consumption of reaction gas.

Graphite or carbon composite materials are widely used for a separatorfor a polymer electrolyte membrane fuel cell. The graphite and thecarbon composite material show strong anti-corrosiveness to theoxidation-reduction reaction of a fuel cell, and also have a merit oflow bulk density in comparison with metallic materials (e.g., stainlesssteel).

When the graphite or the carbon composite material is used as a materialfor a separator, according to the prior art, a resin such asthermosetting or thermoplastic resin is usually added to the separatormaterial in order to prevent movement of hydrogen by filling microporesof the separator, and also for easy forming during the molding process.However, the resin included in the separator causes an increase ofvolume resistance with respect to movement of electrons, and therebydeteriorates performance of a fuel cell. Furthermore, the resinincreases contact resistance between cells.

As an exemplary method for reducing an increase of contact resistancebetween cells cause by the resin in the separator, European PatentPublication No. EP1253657A1 discloses a method in which rib surfaces offlow fields of a separator are etched in an alkaline solution such thatthe resin in the surface area of the rib may be removed.

According to the prior art, the manufacturing process for a stable anduseful separator using a graphite or a carbon composite material hasbeen very complex, non-productive, and non-efficient. Therefore, if aseparator using graphite or a carbon composite material can result inhigher performance and be appropriate for mass production, it promises asubstantial decrease in production cost of a separator and in turnproduction cost of a fuel cell, as well as enhancement of performance ofa separator.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the background of the inventionand should not be taken as an acknowledgement or any form of suggestionthat this information forms the prior art that is already known in thiscountry to a person of ordinary skill in the art.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in an effort to solve theabove-described problem. The motivation for the present invention is toprovide a separator for a fuel cell, a manufacturing method thereof, anda fuel cell including such a separator, providing an enhanced waterexhaust performance, enhanced durability, and being more appropriate formass production.

From such motivation, a separator for a fuel cell according to thepresent invention is a separator for a fuel cell that is capable ofclosely contacting either an anode or a cathode of an MEA (membraneelectrode assembly) of a fuel cell and interposing a fluid diffusionlayer, the separator having a flow field channel for allowing a fluid toflow between the separator and the fluid diffusion layer, characterizedin that: the separator comprises a lamellar structure graphite foil; anda hydrophobic layer is formed by impregnation on an interior side of theflow field channel.

The lamellar structure graphite foil may include a stainless steel layertherewithin. Preferably in this case, the stainless steel layer isexteriorly exposed, interposing the hydrophobic layer.

Preferably, the graphite foil is substantially free from thermosettingor thermoplastic resin.

A bulk density of the graphite foil preferably lies in the range of 1.5g/cm³ to 2.0 g/cm³.

A thickness of the graphite foil preferably lies in the range of 0.5 mmto 3 mm.

A thickness of the hydrophobic layer preferably lies in the range of 30μm to 100 μm.

It is preferable that at least one manifold is formed in the separator,and a sealing member is unified to the separator along eachcircumference of the at least one manifold and an area for contactingthe fluid diffusion layer.

It is preferable that the sealing member encloses, respectively along aclosed curve, each of the at least one manifold and the area forcontacting the fluid diffusion layer.

In addition, a method for manufacturing a separator according to thepresent invention is a method for manufacturing a separator for a fuelcell that is capable of closely contacting either an anode or a cathodeof an MEA of a fuel cell and interposing a fluid diffusion layer, andhas a flow field channel for allowing a fluid to flow between theseparator and the fluid diffusion layer, characterized in that themethod includes:

preparing a graphite foil of a predetermined size;

forming a mask pattern on the graphite foil corresponding to the flowfield channel;

forming the flow field channel on the graphite foil by etching thegraphite foil formed with the mask pattern thereon;

forming a hydrophobic layer on an interior side of the flow fieldchannel by impregnation; and

removing the mask pattern from the graphite foil.

It is preferable that the forming of a mask pattern on the graphite foilincludes:

coating the graphite foil with a dry film resist;

exposing the coated graphite foil; and

developing the dry film resist on the graphite foil by moving a spraynozzle of a spray-type developing apparatus thereover.

As another example, it is also preferable that the forming of a maskpattern on the graphite foil includes attaching a mask on the graphitefoil, the mask being provided with a pattern corresponding to the flowfield channel and being made of rubber or stainless steel.

It is preferable that the forming of the flow field channel on thegraphite foil includes at least one of sandblasting and ultrasonicetching.

It is preferable that the forming of a hydrophobic layer on the interiorside of the flow field channel by impregnation includes:

forming a hydrophobic layer on the graphite foil attached with the maskpattern and formed with the flow field channel; and

drying the graphite foil formed with the hydrophobic layer, in atemperature range of 50° C.-90° C.

It is preferable that, in the forming of a hydrophobic layer on thegraphite foil, a hydrophobic solution is spray coated on a surface ofthe graphite foil, or the graphite foil is dipped in the hydrophobicsolution.

As for a bipolar separator in which the flow field channel is formed oneach of front and rear sides thereof, it is preferable that:

the mask pattern includes a front mask pattern and a rear mask pattern;

at least one pair of aligning holes are formed at each of the front andrear mask patterns;

at least one aligning hole is formed through the graphite foilcorresponding to the aligning holes of the mask patterns; and

the aligning holes of the mask patterns and the aligning holes of thegraphite foil are aligned by using at least one pair of aligning barscorresponding thereto.

It is preferable that the at least one pair of aligning holes and the atleast one pair of aligning bars respectively include a plurality ofpairs thereof, corresponding to different sizes.

A fuel cell stack according to the present invention is a fuel cellstack including at least one unit cell, wherein the at least one unitcell includes:

an MEA including a polymer electrolyte membrane, and an anode and acathode formed on both sides thereof;

a pair of fluid diffusion layers contiguously disposed to the anode andthe cathode at both sides of the MEA; and

a pair of separators for closely contacting the pair of fluid diffusionlayers, forming flow field channels on sides thereof facing the fluiddiffusion layers so as to form a reaction region, and forming manifoldregions peripheral to the reaction region,

characterized in that

at least one of the pair of separators includes a lamellar structuregraphite foil, and

a hydrophobic layer is formed by impregnation on an interior side of theflow field channels of the at least one of the pair of separators.

The lamellar structure graphite foil may include a stainless steel layertherewithin. Preferably in this case, the stainless steel layer isexteriorly exposed, interposing the hydrophobic layer.

Preferably, the graphite foil is substantially free from thermosettingor thermoplastic resin.

A bulk density of the graphite foil preferably lies in the range of 1.5g/cm³ to 2.0 g/cm³.

A thickness of the graphite foil preferably lies in the range of 0.5 mmto 3 mm.

A thickness of the hydrophobic layer preferably lies in the range of 30μm to 100 μm.

It is preferable that a sealing member is unified to the separator alongeach circumference of the manifold and the reaction region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an exemplary fuel cell stack.

FIG. 2 is an exploded perspective view of a fuel cell stack according toan embodiment of the present invention.

FIG. 3 a sectional view showing a detailed junction structure of an MEAand fluid diffusion layers in a unit cell of a fuel cell stack accordingto an embodiment of the present invention.

FIG. 4 shows a front side 400 (i.e., a side toward a cathode) of acathode side separator 260 of a unit cell 200 of a fuel cell stackaccording to an embodiment of the present invention.

FIG. 5 shows a rear side 500 (i.e., a side opposite to a cathode) of acathode side separator 260 of a unit cell 200 of a fuel cell stackaccording to an embodiment of the present invention.

FIG. 6 shows a rear side 600 (i.e., a side toward an anode) of an anodeside separator 250 of a unit cell 200 of a fuel cell stack according toan embodiment of the present invention.

FIG. 7 is a sectional view of FIG. 4 along a line VII-VII.

FIG. 8 is a sectional view of FIG. 6 along a line VIII-VIII.

FIG. 9 is a flowchart showing a method for manufacturing a separator fora fuel cell according to an embodiment of the present invention.

FIG. 10 is a drawing for illustrating a process for attaching a mask toa graphite foil in a method for manufacturing a separator according toan embodiment of the present invention.

FIG. 11 is a sectional view of FIG. 6 along a line XI-XI according to asecond embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will hereinafter be described indetail with reference to the accompanying drawings.

In the description below, terms such as upper portion/lower portion andfront side/rear side implying directions are used for convenience ofexplanation. However, such terms are only for convenience of descriptionand better understanding, so it should not be understood that specificelements should be disposed at such directions according to the presentinvention.

FIG. 2 is an exploded perspective view of a fuel cell stack according toan embodiment of the present invention.

As shown in FIG. 2, a fuel cell stack according to an embodiment of thepresent invention includes a stack of a plurality of unit cells 200. Thestack of the unit cells 200 are conjoined by end plates 290 disposed atends thereof. The end plates 290 are formed with current collectors 295,and thereby supply electricity produced through the entire fuel cellstack.

At the end plate 290 applying a conjoining pressure to the fuel cellstack, a multiplicity of connection holes 270 are formed for supplyingand exhausting reaction gas etc. to/from the fuel cell stack. Theconnection holes 270 include a hydrogen supply hole 271, a coolantsupply hole 272, an air supply hole 273, an air exhaust hole 274, acoolant exhaust hole 275, and a hydrogen exhaust hole 276. Each of theconnection holes 271-276 is connected to a corresponding manifold in thefuel cell stack 100.

For each unit cell 200, fluid diffusion layers 225 are attached to frontand rear of an MEA 230, and separators 250 and 260 are disposed to frontand rear of the MEA 230 and attached to the fluid diffusion layers 225.Hereinafter, the left side of FIG. 2 is referred to as frontward of theunit cells 200, and the right side of FIG. 2 is referred to as rearwardof the unit cells 200.

The above-described fluid diffusion layer 225 is usually called a gasdiffusion layer (GDL) in the art. However, the materialdiffused/distributed through the fluid diffusion layer 225 is notnecessarily a gas, so the term fluid diffusion layer is hereinafter usedinstead of the usual term gas diffusion layer.

FIG. 3 is a sectional view showing a detailed junction structure of anMEA 230 and fluid diffusion layers 225 in a unit cell 200 of a fuel cellstack according to an embodiment of the present invention.

As shown in FIG. 3, according to an embodiment of the present invention,an anode 221 and a cathode 222 are respectively formed, by pressing, tofront and rear sides of a polymer electrolyte membrane 210, and thefluid diffusion layer 225 is formed to each exterior of the anode 221and the cathode 222

Referring back to FIG. 2, a cathode side separator 260 is in closecontact with the cathode of the MEA 230 having the fluid diffusion layer225, and an anode side separator 250 is in close contact with the anode.

In the following description of an embodiment of the present invention,the anode side separator 250 is described to be of a monopolarstructure, and the cathode side separator 260 is described to be of abipolar structure. However, the protection scope of the presentinvention should not be understood to be limited thereto, because thespirit of the present invention may be applied to various other schemesof disposing separators at the front and rear of the MEA 230.

The separators 250 and 260 are closely conjoined to exterior surfaces ofthe fluid diffusion layers 225, and have a plurality of flow fieldchannels on their surface closely facing the fluid diffusion layers. Theflow field channels of the separators 250 and 260 are used as passagesof reaction gas between the fluid diffusion layers 225 and theseparators 250 and 260.

The separators 250 and 260 distribute reaction gases through the fuelcell stack, separate a fuel gas and a reduction gas, and electricallyconnect adjacent unit cells by providing an electron passage between ananode and a cathode of adjacent unit cells. In addition, the separatorshave a heat exhaust structure for exhausting heat produced by theoxidation-reduction reaction of the fuel cell stack, and providemechanical strength for supporting the stacked unit cells.

The separators 250 and 260 according to an embodiment of the presentinvention are hereinafter described in further detail.

FIG. 4 shows a front side 400 (i.e., a side toward a cathode) of acathode side separator 260 of a unit cell 200 of a fuel cell stackaccording to an embodiment of the present invention.

As shown in FIG. 4, at an upper portion of the cathode side separator260, a hydrogen supply manifold 471, a coolant supply manifold 472, andan air supply manifold 473 are formed through the separator 260,respectively corresponding to the hydrogen supply hole 271, the coolantsupply hole 272, and the air supply hole 273.

In addition, at a lower portion of the cathode side separator 260, anair exhaust manifold 474, a coolant exhaust manifold 475, and a hydrogenexhaust manifold 476 are formed through the separator 260, respectivelycorresponding to the air exhaust hole 274, the coolant exhaust hole 275,and the hydrogen exhaust hole 276.

For preventing leakage of reaction gas/coolant from the manifolds471-476, manifold sealant grooves 481-486 for application of sealant arerespectively formed around each of the manifolds 471-476.

At a fuel cell reaction region (i.e., a region contacting the fluiddiffusion layer 225) in front of the cathode side separator 260, airflow-field channels 410 for supplying air to the MEA 230 are formed byribs 420 defining the route. Such air flow-field channels 410 are formedas grooves of a predetermined depth.

FIG. 4 illustrates that the air flow-field channels 410 of the separator260 according to an embodiment of the present invention are of aserpentine shape. However, this is only an exemplary shape of which thespirit of the present invention may be applied, and accordingly theprotection scope of the present invention should not be understood to belimited thereto.

At entry ends of the air flow-field channels 410, an air supply hole 450for supplying air to the air flow-field channels 410 is formed throughthe separator 260, and at exit ends of the air flow-field channels 410,air exhaust hole 460 for exhausting air from the air flow-field channels410 is formed through the separator 260.

The air supplied to the air supply manifold 473 is supplied to the airsupply hole 450 through air supply passages 550 (refer to FIG. 5) formedat the rear side of the separator 260. The air exhausting from the airexhaust hole 460 is exhausted to the air exhaust manifold 474 throughair exhaust passages 560 (refer to FIG. 5) formed at the rear side ofthe separator 260.

Around the reaction region having the air flow-field channels 410, theair supply hole 450, and the air exhaust hole 460, a reaction regionsealant groove 480 is formed for application of sealant for preventingair leakage from the reaction region.

The sealant grooves 480-486 are in the form of closed loops thatrespectively enclose the manifolds and the region closely contacting thefluid diffusion layer.

In an area between sealant grooves 480-486 on the separator 260, atleast one pair of aligning holes are formed through the separator 260,for alignment of flow field channels on front and rear sides of theseparator 260.

The at least one pair of aligning holes are formed as a plurality ofaligning hole pairs (pairs of 431 and 432, and 433 and 434) havingdifferent sizes. By forming the aligning hole pairs with differentsizes, larger diameter aligning holes 431 and 432 enables roughalignment (i.e., allowing easy alignment), and smaller diameter aligningholes 433 and 434 enable a precise alignment.

As an example, FIG. 4 illustrates that the pair of the larger diameteraligning holes 431 and 432 are formed at upper and lower portions of theseparator, and the pair of the smaller diameter aligning holes 433 and434 are formed interior to the larger diameter aligning holes 431 and433.

Again as an example, FIG. 4 illustrates that the aligning holes 431 and433 at the upper portion are formed between the sealant groove 481 ofthe hydrogen supply manifold 471 and the sealant groove 482 of thecoolant supply manifold 472, and the aligning holes 432 and 434 at thelower portion are formed between the sealant groove 486 for the hydrogenexhaust manifold 476 and the sealant groove 485 for the coolant exhaustmanifold 475.

Aligning of the flow field channels on the front and rear sides 400 and500 of the bipolar separator 260 using the aligning holes 431, 432, 433,and 434 is described in further detail in the description regarding amanufacturing method of a separator according to an embodiment of thepresent invention.

FIG. 5 shows a rear side 500 (i.e., a side opposite to a cathode) of acathode side separator 260 of a unit cell 200 of a fuel cell stackaccording to an embodiment of the present invention.

As shown in FIG. 5, the air supply manifold 473 and the air supply hole450 are interconnected through the air supply passages 550, and the airexhaust manifold 474 and air exhaust hole 460 are interconnected throughthe air exhaust passages 560. The air supply passages 550 and the airexhaust passages 560 are formed on the rear side 500 of the separator260 in the form of grooves.

In addition, hydrogen supply passages 555 are formed to be connected tothe hydrogen supply manifold 471, and hydrogen exhaust passage 565 areformed to be connected to the hydrogen exhaust manifold 476. Thehydrogen supply passages 555 and hydrogen exhaust passage 565 are formedon the rear side 500 of separator 260 in the form of grooves.

Sealant grooves 581, 583, 584, and 586 for application of sealant arerespectively formed around each of the manifolds 471, 473, 474, and 476,in the form of closed loops that respectively enclose the manifolds andpassages connected thereto.

At a central portion on the rear side 500 of the separator, coolantflow-field channels 510 for circulating the coolant supplied from thecoolant supply manifold 472 are formed by ribs 520 defining the route.Such coolant flow-field channels 510 on the rear side 500 of theseparator are formed to be aligned with the air flow-field channels 410on the front side 400 of the separator.

The fuel cell coolant is supplied to the coolant flow-field channels 510through the coolant supply manifold 472, is circulated on the rear sideof the separator 260, and is then exhausted through the coolant exhaustmanifold 475.

A sealant groove 580 for application of sealant for preventing leakageof coolant is formed enclosing the coolant supply manifold 472, thecoolant flow-field channel 510, and the coolant exhaust manifold 475.

FIG. 6 shows a rear side 600 (i.e., a side toward an anode) of an anodeside separator 250 of a unit cell 200 of a fuel cell stack according toan embodiment of the present invention.

The rear side of the anode side separator 250 is structured similarly tothe front side 400 of the cathode side separator 260.

That is, as shown in FIG. 6, at an upper portion of the separator 250, ahydrogen supply manifold 671, a coolant supply manifold 672, and an airsupply manifold 673 are formed through the separator 250. In addition,at a lower portion of the separator 250, an air exhaust manifold 674, acoolant exhaust manifold 675, and a hydrogen exhaust manifold 676 areformed through the separator 250. In addition, manifold sealant grooves681-686 for application of sealant are respectively formed around eachof the manifolds 671-676.

At a fuel cell reaction region in rear of the anode side separator 250,hydrogen flow-field channels 610 for supplying hydrogen to the MEA 230are formed by ribs 620 defining the route.

At entry ends of the hydrogen flow-field channels 610, a hydrogen supplyhole 650 for supplying hydrogen to the hydrogen flow-field channels 610is formed through the separator 250, and at exit ends of the hydrogenflow-field channels 610, a hydrogen exhaust hole 660 for exhaustinghydrogen from the hydrogen flow-field channels 610 is formed through theseparator 260.

The hydrogen supplied to the hydrogen supply manifold 671 is supplied tothe hydrogen supply hole 650 through hydrogen supply passages 555 (referto FIG. 5) formed at the rear side of the cathode side separator 260 ofan adjacent unit cell. The hydrogen exhausting from the hydrogen exhausthole 660 is exhausted to the hydrogen exhaust manifold 676 throughhydrogen exhaust passages 565 (refer to FIG. 5) formed at the rear sideof the cathode side separator 260 of an adjacent unit cell.

Around the reaction region having the hydrogen flow-field channels 610,the hydrogen supply hole 650, and the hydrogen exhaust hole 660, areaction region sealant groove 680 is formed for application of sealantfor preventing hydrogen leakage from the reaction region.

Aligning holes 631, 632, 633, and 634 are formed through the anode sideseparator 250 at the same positions and same sizes with the aligningholes 431, 432, 433, and 434 of the cathode side separator 260. Stackingof the unit cells 200 may be eased due to the aligning holes 431, 432,631, and 632.

In the above description, exterior features of and reaction gas/coolantcommunication through the separators 250 and 260 according to anembodiment of the present invention were main topics.

Hereinafter, materials, interior structure, and manufacturing method ofseparators 250 and 260 are described in detail.

FIG. 7 is a sectional view of FIG. 4 along a line VII-VII, and FIG. 8 isa sectional view of FIG. 6 along a line VIII-VIII.

According to an embodiment of the present invention, graphite foil 700and 800 is used as a material for the separators 250 and 260.

Manufacturing of a graphite foil is usually accompanied by a pressingstep, and a graphite foil pressed with a high pressure has an internallamellar structure.

Therefore, according to an embodiment of the present invention, thematerial for the separators 250 and 260 may be called a lamellarstructure graphite foil.

The enlarged portion A of FIG. 7 and portion B of FIG. 8 may be referredto for the lamellar structure of the graphite foils 700 and 800 used formaterials of the separators 250 and 260 according to an embodiment ofthe present invention,

For a usual graphite foil, heat conductivity thereof is more than 250W/mK in the stacking direction of unit cells. Therefore, it maycontribute to an enhancement of heat exhaust performance of a fuel cellstack and also to uniformity in temperature distribution of a fuel cellstack.

In a separator of a graphite/carbon composite material according to theprior art, a resin such as thermosetting resin or thermoplastic resin isincluded in the separator, for preventing leakage of reaction gas or foreasier molding of graphite material.

However, according to the separators 250 and 260 according to anembodiment of the present invention, the graphite foil used for theseparators is substantially free from such resin. Lamellar structuregraphite foils are currently mass produced, and if the resin of theprior art does not need to be contained, mass productivity of thegraphite foils further increases. Therefore, if such lamellar structuregraphite foil is used for manufacturing a separator, it contributes to areduction of production cost of a separator.

Furthermore, the separator of graphite material containing resinaccording to the prior art is rarely used for a fuel cell operating at atemperature higher than about 100° C. because heat deflectiontemperature of most of the resins is below about 100°C.

Therefore, in the case that a graphite foil free from the resin may beused for a separator, an operating temperature range of a fuel cell maybe broadened, and durability of the fuel cell may be enhanced againstthe case that the fuel cell is overheated more than 100° C.

In fact, separators 250 and 260 may have high formability for, e.g.,forming flow fields on the separators 250 and 260, without containingresin therein. Furthermore, leakage prevention of reaction gases and ahydrophobic effect may be preserved, while enhancing the durability. Amethod for manufacturing a stable and high-performance separator using agraphite foil without containing resin is described later.

A separator of a fuel cell should maintain uniform pressure withoutdeformation during stacking of a fuel cell stack, and it should providesufficient electric conductivity.

Therefore, it is preferable that the graphite foil used as a materialfor the separators 260 and 250 of an embodiment of the present inventionshows a bulk density higher than 1.5 g/cm³. As an experimental result,in the case that the bulk density of the graphite foil is lower than 1.5g/cm³, reaction gas leakage or a contact resistance increase has beenfound to possibly occur due to excessive deformation of the separatorwhen stacked in the fuel cell stack.

In addition, an excessively high bulk density causes an increase ofproduction cost of a graphite foil, i.e., production cost of aseparator. Therefore, graphite foil used for separators 250 and 260 ofan embodiment of the present invention may have a bulk density less than2.0 g/cm³.

In the case that a separator is excessively thin, performance anddurability of a fuel cell stack may be deteriorated due to an increaseof gas permeability of the separator. Considering such a point, it ispreferable that thicknesses D1 and D2 of graphite foils used for theseparators 250 and 260 according to an embodiment of the presentinvention are greater than 0.5 mm.

A minimum depth for the flow field channels is usually about 0.2 mm.Therefore, in the case that the thickness of a graphite foil is lessthan 0.5 mm, the thickness d2 of the flow field channels becomes lessthan 0.3 mm. In this case, it has been found that gas permeability ofthe graphite foils 700 and 800 becomes excessively high.

Therefore, by forming the separator of graphite foil at a thickness ofat least 0.5 mm, enhancement of durability and performance is expected,guaranteeing such minimum thickness.

However, if the separator becomes excessively thick, production costincreases without any increase in performance and durability. As anexperimental result, it has been found that the thickness of thegraphite foil used for the separator according to an embodiment of thepresent invention does not need to be more than 3 mm.

Since a normal operating temperature of a polymer electrolyte membranefuel cell is less than 100° C. and about 80° C., efficient and steadyexhaust of reaction products, i.e., heat and water, is very important.

For such purpose, hydrophobic layers 710 and 810 are formed byimpregnation on interior surfaces of the flow field channels 410 and 610of the separators 260 and 250 according to an embodiment of the presentinvention. The hydrophobic layers formed on the flow field channels 410and 610 also contribute to efficient supplying of reaction gases toelectrodes through the fluid diffusion layers 225.

As shown in FIGS. 7 and 8, the hydrophobic layers 710 and 810 are notformed on the surfaces of the ribs 420 and 620. Therefore, contactresistance between the fluid diffusion layers 225 and the separators 260and 250 does not increase because of the hydrophobic layers 710 and 810.

For conventional hydrophobic treatment according to the prior art, ahydrophobic layer is simply coated on a surface of a workpiece. In thiscase, the coated hydrophobic layer may be easily scraped or removed.

However, according to hydrophobic layers 710 and 810 of an embodiment ofthe present invention, interior sides of the flow field channels 410 and610 are firstly modified to a roughness of several decades ofmicrometers (μm), and then the hydrophobic layers 710 and 810 are formedby impregnation on the roughened surface. Therefore, since hydrophobiclayers 710 and 810 are formed on the roughened surface, cohesionstrength hydrophobic layers 710 and 810 to the interior surface of theflow field channels 410 and 610 is enhanced and thereby durability ofthe hydrophobic layers 710 and 810 is enhanced.

Thickness of the hydrophobic layers 710 and 810 formed by impregnationinto the flow field channels preferably lies in a range of 30 μm to 100μm. The thickness of the hydrophobic layers 710 and 810 is preferablyabove 30 μm to durably provide the hydrophobic effect of the hydrophobiclayer 710 and 810, considering the reaction gases and the coolant flowunder the conjoining force of the fuel cell stack. To the contrary, inthe case that the hydrophobic layers 710 and 810 are thicker than 100μm, the hydrophobic solution used for the hydrophobic layer isexcessively impregnated into the flow field channels 410 and 610. Inthis case, dividing of lamellas may occur due to, e.g., bubbles formedat the lamellar structure graphite foil, when heat treatment forhydrophobic effect (for example, heat treatment at 330° C.).

As has already been described with reference to FIGS. 4 to 6, sealantgrooves 481-486, 681-686, 480, and 680 are formed around the manifolds471-476 and 671-676 and the area contacting the fluid diffusion layers.

As shown in FIGS. 7 and 8, sealing members 790 and 890 are applied tothe sealant grooves of the separators 260 and 250, and become integralwith the separators 260 and 250. FIGS. 7 and 8 illustrate that sealingmembers 790 and 890 are applied only to specific portions, however, thesealing member 790 and 890 are actually applied to each of the sealantgrooves 480-486, 580-586, and 680-686 of the separators 260 and 250. Arubber liquid of, e.g., silicon series, fluorine series, and olefinseries may be used as the sealing members 790 and 890.

Since the sealing member is applied to the separators 260 and 250 on thesealant groove and becomes integral therewith, the assembly process of afuel cell stack may be simplified.

Hereinafter, an embodiment of the present invention is described indetail regarding a method for manufacturing a separator made of alamellar structure graphite foil and having a hydrophobic layer formedby impregnation on its flow field channels.

FIG. 9 is a flowchart showing a method for manufacturing a separator fora fuel cell according to an embodiment of the present invention.

As shown in FIG. 9, according to a method for manufacturing a separatorfor a fuel cell according to an embodiment of the present invention, agraphite foil of a predetermined thickness and bulk density is firstlyprepared at step S910.

Then at step S920, a mask pattern is formed on the prepared graphitefoil corresponding to manifolds, sealant grooves, and flow fieldchannels.

Then at step S930, the manifolds, the sealant grooves, and the flowfield channels are formed on the graphite foil by selectivelydry-etching the graphite foil formed with the mask pattern thereon.

Subsequently at step S940, a hydrophobic layer is formed on eachinterior side of the flow field channels by impregnation.

Finally at step S950, the mask pattern is removed from the graphite foilhaving the impregnated hydrophobic layer, and then the graphite foil iscleaned and heat-treated such that a separator is finalized.

A monopolar separator is finished by such a process.

For a bipolar separator, flow-field channels etc. (i.e., the manifolds,sealant grooves, and the flow field channels) are formed by executingthe steps of S920 and S930 with respect to one side of the preparedgraphite foil, and then the steps of S920 and S930 are also executed tothe other side of the graphite foil such that the flow-field channelsetc. are also formed to the other side.

Then, the graphite foil having the flow-field channels etc. on bothsides thereof is processed according to the steps of S940 and S950, andaccordingly the bipolar separator is completed.

A manufacturing method for a bipolar separator will be obviouslyunderstood from a description of a manufacturing method for a monopolarseparator.

Hereinafter, the above-mentioned steps S910-S950 are described infurther detail, with respect to an exemplary monopolar separator 250.

Firstly at step S910 of preparing the graphite foil, the preparedgraphite foil has a thickness of 1.0 mm, an overall density of 1.78g/cm3, and dimensions of 10 cm×15 cm. Moisture is removed from theprepared graphite foil by drying the foil in a dryer for 5 minutes at100° C. The aligning holes 631, 632, 633, and 634 are formed at theprepared graphite foil. In this stage, only the diameters of thealigning holes 631, 632, 633, and 634 and the distance therebetween needbe paid attention to in order to be formed in accordance with apredetermined specification, since the flow-fields channels etc. are notyet formed thereon.

The step S920 of forming the mask pattern may be further embodied invarious ways. For example, a mask of a rubber or stainless steelmaterial with a pattern corresponding to the flow field channels etc.may be attached to the graphite foil.

For another example, a dry film may be used.

In the case that a dry film (for example, BF410) is used, a dry film(e.g., BF410) of a thickness of 100 μm is coated on the graphite foilusing a laminator apparatus. For the laminating process, it ispreferable that the upper roller temperature is about 70° C., the lowerroller temperature is about 65° C., and the rolling speed is about 60mm/sec.

A film mask formed with a pattern for flow-field channels etc. is laidin contact with the graphite foil, and then the graphite foil with thefilm mask is exposed at an exposure machine for about 18-23 seconds withan energy density of about 20 mW/cm2.

In the case of a dry film, it may bulge when it is dipped in a liquiddeveloper. Therefore, in order for develop a graphite foil coated with adry film, it is preferable that a spray nozzle moves while spraying aliquid developer (e.g., 1˜2% solution of Na2CO3) in a spray-typedeveloping machine.

The developing conditions of the spray-type developing machine ispreferably set to be the temperature of the liquid developer being about25° C., the spray pressure being about 2.7 Kg/cm2, and the nozzle movingspeed being about 80 mm/sec. When developed as such, it is preferablybaked in an oven for about 5 minutes at about 100° C.

The step S920 of forming a mask pattern includes a step of attaching themask on the graphite foil. For such a purpose, the aligning holes 631,632, 633, and 634 and aligning bars are used. Attaching the mask on thegraphite foil using the aligning holes 631, 632, 633, and 634 ishereinafter described in further detail.

As shown in FIG. 10, a pair of larger diameter aligning bars 1021 and1022 and a pair of smaller diameter aligning bars 1023 and 1024 areformed protruding on a working plate 1100 on an exposure machine 1000.The working plate 1100 is formed as a transparent plate such that anexposure light generated at a light source 1005 of the exposure machine1000 may penetrate therethrough.

A prepared mask 1010 is formed with aligning holes 1011, 1012, 1013, and1014 at positions corresponding to the aligning bars 1021, 1022, 1023,and 1024. In addition, at the step S910 of preparing the graphite foil,aligning holes 631, 632, 633, and 634 are formed at the graphite foil atpositions corresponding to the aligning bars 1021,1022,1023, and 1024.

Since a thickness of the mask 1010 is usually less than 0.2 mm, and itsuffices that a protruding height h of the aligning bars 1021, 1022,1023, and 1024 from the working plate 1100 is 0.2 mm. The largerdiameter aligning bars 1021 and 1022 protrude slightly more than thesmaller diameter aligning bars 1023 and 1024. Therefore, a separator maybe precisely aligned in the correct position using the smaller diameteraligning bars 1023 and 1024 after it is positioned in a rough alignmentposition using the larger diameter aligning bars 1021 and 1022.

The mask 1010 is laid on the working plate 1100 such that the protrudedaligning bars 1021, 1022, 1023, and 1024 may be inserted into thealigning holes 1011, 1012, 1013, and 1014. In this case, the aligningbars 1021, 1022, 1023, and 1024 fully penetrate the mask 1010 andprotrude therefrom, since the protruding height h of the aligning bars1021, 1022, 1023, and 1024 is greater than the thickness of the mask1010.

Therefore, front and rear sides of the separator may be preciselyaligned by simply disposing the prepared graphite foil such that thealigning holes 631, 632, 633, and 634 are inserted by aligning bars1021, 1022, 1023, and 1024. By attaching masks on the graphite foilaccording to such an aligning process, front and rear sides of theseparator are precisely aligned.

At the step S930 of forming the flow field channels, the graphite foilprocessed with the mask pattern process is dry-etched using, e.g., asand blast apparatus, such that the depth of the flow field channels maybecome about 500 μm.

The sand blast condition is preferably set to be the separator movingspeed being about 40 mm/minute, the nozzle moving speed being about 20m/minute, the spraying pressure of the nozzle being about 3.0kg/cm2, andthe distance between the separator and the nozzle being about 60 mm. Forexample, SiC may be used as an abrasive such that surface roughness onthe bottom of the flow field channels may become several decades ofmicrometers.

An ultrasonic etching process, that is, an etching process using anabrasive, may be performed instead of or together with the sand blastprocess.

At the step S950 of forming the hydrophobic layer by impregnation, thegraphite foil having the flow-field channels and having the mask patternattached thereon is dried in a range of 50° C. to 90° C. after dippingit in a hydrophobic solution (e.g., a 20% PTFE (polytetrafluoroethylene)solution) for 2 to 4 seconds, or processing by spray coating.

At the step S950 of finishing the separator, the mask pattern is removedin acetone using an ultrasonic cleaner such that the dry film attachedto the graphite foil is removed, and then the graphite foil is cleanedand heat-treated to finalize the separator. The process of cleaning andheat-treating the graphite foil is obvious to a person of ordinary skillin the art. As for a mask of a rubber or a stainless steel material, itmay be removed by simply dividing it.

In the above description of an embodiment of the present invention, aseparator is described to use a graphite foil having a uniformstructure. However, the protection scope of the present invention shouldnot be understood to be limited thereto, since variations of embodimentsmay be possible, such as the case that a stainless steel layer isinserted inside the graphite foil used for the separator.

Hereinafter, a second embodiment of the present invention related to thecase that the graphite foil used for the separator includes a stainlesssteel layer therein is described with reference to FIG. 11.

FIG. 11 is a sectional view of FIG. 6 along a line XI-XI according to asecond embodiment of the present invention.

As shown in FIG. 11, a lamellar structure graphite foil 1140 used for aseparator 1150 according to a second embodiment of the present inventionincludes a stainless steel layer 1160 therein.

Such graphite foil 1140 including the stainless steel layer 1160 may beeasily manufactured e.g., by pressing graphite layers of predeterminedthickness to front and rear of the stainless steel layer 1160.

The stainless steel layer 1160 may be of, e.g., SUS304 or SUS316, andits thickness t is preferably 0.1 mm to 0.3 mm. The thickness d3 of thegraphite layers formed at front and rear sides of the stainless steellayer 1160 is preferably more than 0.2 mm considering the depth offlow-field channels 610, and is preferably less than 3 mm consideringthe production cost.

That is, the separator 1150 according to such a second embodiment of thepresent invention uses the graphite foil 1140 formed with, e.g., thestainless layer 1160 of 0.1 mm thickness at its center and graphitelayers of 0.2 mm thickness at each of front and rear sides of thestainless layer 1160 (therefore, total thickness D3 of the graphite foilbecomes 0.5 mm). The flow-field channels 610 are formed by dry-etchingone side (e.g., upper side in FIG. 11) of the graphite foil 1140.

A method for manufacturing the separator 1150 using the graphite foil1140 including the stainless steel layer 1160 therein is the same as amethod for manufacturing the separators 250 and 260 according to thefirst embodiment of the present invention that has been described withreference to FIG. 10.

In the process of dry-etching for forming the flow-field channels 610,the graphite layer is etched such that the stainless steel layer 1160 isexposed. As for the process of dry-etching (e.g., the above-describedsand blasting etching process), the stainless steel layer 1160 has asubstantially smaller etch rate than the graphite layers formed to frontand rear of the stainless steel layer 1160. Therefore, such stainlesssteel layer 1160 functions as an etch stop in the process of etching.

According to such a feature, the manufacturing process of a separatorbecomes easier, and a manufacturing device of lower precision may beused to produce a separator with equally high performance and precision.

Although rigidity of the stainless steel layer 1160 is higher than thatof the graphite layer, the abrasive used in the etching process mayeasily form a roughness thereon, e.g., to the degree of several decadesof micrometers described in connection with the first embodiment.

In addition, in the process of forming the hydrophobic layer (refer tothe step S940 in FIG. 9), the hydrophobic layer 810 is formed while thestainless steel layer 1160 is exposed in the flow-field channels 610.Therefore, as for a finished separator 1150, the stainless steel layer1160 becomes exteriorly exposed interposing the hydrophobic layer 810,since the hydrophobic layer 810 is formed while the stainless steellayer 1160 is exposed in the flow-field channels 610.

In the above description, the present invention has been exemplarilydescribed in connection with a polymer electrolyte fuel cell. However,the protection scope of the present invention should not be understoodto be limited thereto. To the contrary, the spirit of the presentinvention may be applied to an arbitrary fuel cell of which a normaloperating temperature lies in a range below a heat deflectiontemperature of the graphite foil (e.g., below 250° C.). As an example,the spirit of the present invention may also be applied to a DMFC(direct methanol fuel cell).

Therefore, in the above description and also in the appended claims, theterms related to a polymer electrolyte fuel cell should not beunderstood to strictly refer to the specific element of the polymerelectrolyte fuel cell.

For example, the term fuel gas should be understood in a collectivemeaning that covers methanol fuel supplied in the form of liquid as wellas hydrogen in the form of gas. In addition, as for a fuel cell otherthan a polymer electrolyte fuel cell, the term MEA should be understoodto be referring to a corresponding element of the fuel cell.

As described above, according to an embodiment of the present invention,lamellar structure graphite foil is used as a material for a separatorfor a fuel cell, and hydrophobic layer is formed by impregnation on aninterior wall of flow field channels of the separator.

Therefore, water produced at cathodes may be efficiently exhausted, andthereby performance of the fuel cell may be enhanced, by, e.g.,accelerating diffusion of reaction gases to catalyst layers.Accordingly, power density per unit volume of a fuel cell stack may beenhanced.

In addition, since the hydrophobic layer is formed by impregnation on aninterior wall of flow-field channels of a separator that has been formedby dry-etching, durability of the hydrophobic layer is enhanced, andconsequently, durability and reliability of a fuel cell is enhanced.

Furthermore, since an interior of the graphite foil is substantiallyfree from resins such as thermosetting or thermoplastic resin and ahydrophobic layer is prevented from forming on the separator surfacecontacting an MEA by a mask pattern, additional processes for reducingcontact resistance between cells are not required.

Furthermore, since such graphite foil exhibits high heat conductivity,the cooling effect of a fuel cell stack is enhanced and temperaturedistribution of a fuel cell stack becomes uniform.

Furthermore, since the lamellar structure graphite foil used as amaterial of a separator may be mass-produced, production cost of aseparator and accordingly a fuel cell may be reduced.

Furthermore, since the manufacturing process of a separator may besimplified and be more adaptive to mass-production, the production costof a separator and accordingly a fuel cell may be reduced.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiments, but, on the contrary, is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims.

Throughout this specification and the claims that follow, unlessexplicitly described to the contrary, the word “comprise” or variationssuch as “comprises” or “comprising” will be understood to imply theinclusion of stated elements but not the exclusion of any otherelements.

What is claimed is:
 1. A separator for a fuel cell that is capable of contacting either an anode or a cathode of an MEA (membrane electrode assembly) of a fuel cell and interposing a fluid diffusion layer, the separator comprising: a flow field channel for allowing a fluid to flow between the separator and the fluid diffusion layer, wherein the flow field channel is defined by ribs and is formed as a groove, a lamellar structure graphite foil; and a hydrophobic layer formed by impregnation on an interior side of the groove of the flow field channel, the interior side of the groove of the flow field channel having a roughness of decades of micrometers; wherein the hydrophobic layer is formed only on the interior side of the groove; and wherein the lamellar structure graphite foil comprises a stainless steel layer therewithin, and wherein the stainless steel layer has a portion that is directly contacting the hydrophobic layer.
 2. The separator for a fuel cell as claimed in claim 1, wherein the graphite foil is substantially free from thermosetting or thermoplastic resin.
 3. The separator for a fuel cell as claimed in claim 1, wherein a bulk density of the graphite foil lies in the range of 1.5 g/cm3 to 2.0 g/cm3.
 4. The separator for a fuel cell as claimed in one of claim 1, 2, or 3, wherein thickness of the graphite foil lies in the range of 0.5 mm to 3 mm.
 5. The separator for a fuel cell as claimed in one of claim 1, 2, or 3, wherein thickness of the hydrophobic layer lies in the range of 30 μm to 100 μm.
 6. The separator for a fuel cell as claimed in one of claim 1, 2, or 3, wherein: at least one manifold is formed in the separator; and a sealing member is unified to the separator along each circumference of the at least one manifold and an area for contacting the fluid diffusion layer.
 7. The separator for a fuel cell as claimed in claim 6, wherein the sealing member encloses, respectively along a closed curve, each of the at least one manifold and the area for contacting the fluid diffusion layer. 